Current mode sense amplifiers (CSAs) have been used in integrated circuits to sense and amplify differential input currents. In applications in semiconductor memory, for example, the CSAs are often used to sense and amplify input currents resulting from reading memory cell data and being provided over long signal lines. As a result, the input currents are typically very weak and low in magnitude. In applications such as these, control of the CSA's loop gain is important because it affects the operating characteristics of the CSA. For example, where the loop gain of a CSA is approximately equal to 1, the dominant mode of operation for the CSA is sensing differential input currents. In contrast, as the loop gain of a CSA increases to be greater than 1, the dominant mode of operation for the CSA transitions from current sensing to behaving as a latch circuit. Thus, controlling loop gain is desirable in order to control the behavior of the CSA.
FIG. 1 illustrates a conventional current mode sense amplifier (CSA) 100. The CSA 100 includes a pair of cross coupled p-channel field effect transistors (PFET) 106, 116 and diode coupled PFET transistors 110, 120 to provide active loads. PFET bias transistors 102, 112 are coupled to the PFET transistors 106, 116 and biased by a bias voltage Vpbias. Differential input currents are applied to the input-output nodes 104, 114 from input-output lines Gio, GioF to be sensed and amplified by the CSA 100. As known, the loop gain of the CSA 100 is gmR, where gm is the transconductance of PFET transistors 106, 116 and R is the load provided by the PFET transistors 110, 120. As also known, the load for the diode coupled PFET transistors 110, 120 is 1/gm. As a result, the loop gain for the CSA 100 is approximately 1, and the loop gain remains substantially constant despite variations in factors affecting gm, such as process, voltage, and temperature (PVT). Although the CSA 100 has the benefit of a being able to maintain a substantially constant loop gain for changes in PVT, for operation a supply voltage Vcc for the CSA 100 should be greater than the sum of the threshold voltages of the transistors 106 (or 116) and transistors 110 (or 120), and a voltage margin for operation. In low voltage, low power systems, however, providing a supply voltage of this level is not desirable.